investigatory projects class 12

1. Aim
To understand the working of NOR gate using npn transistor, p-n junction
diodes and to design a circuit of NOR gate using npn transistor, p-n junction
diode, LED, push button type toggle switches and suitable resistances.

2. Introduction
NOR gate is an Boolean operator based on the mathematical function of
complement of addition.
It works on digital signal as its input which have only two states of voltages, low
as 0V and high as 5V.
NOR gate consists of electonic circuit of two p-n junction diodes and npn
transistor having two input digital signals which can take four different
combinations on voltages, for each combination of voltages there is a
predefined output according to complement of addition rule.

3. Apparatus used
p-n junction diode (IN 4007)
Npn transistor (547 b)
Circuit board
Wires
9V battery and battery connector
LED (5mm, 1.5V)
Resistors
Push button type toggle switch
Push buttons

4. Theory and working
1.)Working of p-n junction diode in forward and reverse biasing

6. 2.)Working of npn transistor in different configurations

7. 3.)Understanding the functioning of NAND gate

9. 4.)Circuit diagram used in making of NOR gate and its working
When the push button is in ‘released state’ then terminal ‘2’ is connected to
terminal ‘3’, when push button is in ‘active or pressed’ then terminal ‘2’ is
connected to terminal ‘1’.
The push button connected to first diode is ‘button 1’ and the other one is
‘button 2’.
When ‘button 1’ is released and ‘button 2’ is released then it is equivalent to
A(0), B(0). In this state, first diode is reverse biased and second diode is reverse
biased, current does not flow through base of transistor, leading to cut-off

10. mode of transistor and current flows from collector to LED leading to high
output.
When ‘button 1’ is pressed and ‘button 2’ is released then it is equivalent to
A(1), B(0). In this state, first diode is forward biased and second diode is reverse
biased, current flows through base of transistor, leading to saturation mode of
transistor and current flows from collector to negative terminal of battery
through transistor, leading to no current through LED and low output.
When ‘button 2’ is pressed and ‘button 1’ is released then it is equivalent to
A(0), B(1). In this state, first diode is reverse biased and second diode is forward
biased, current flows through base of transistor, leading to saturation mode of
transistor and current flows from collector to negative terminal of battery
through transistor, leading to no current through LED and low output.
When ‘button 1’ is pressed and also ‘button 2’ is pressed then it is equivalent to
A(1), B(1). In this state, first diode and second diode is reverse biased, current
flows through base of transistor, leading to saturation mode of transistor and
current flows from collector to negative terminal of battery through transistor,
leading to no current through LED and low output.

11. Conclusion
The output of NOR gate for different combinations inputs clearly shows the
expected output according to the truth table.
Sources of errors
Write yourself
Precautions
Do not connect any electronic component without resistance in series, it can
burn the component.
Bibliography
NCERT textbook of class 12
Physics by S.L.Arora
Circuits available on internet